The present invention relates to a multicolor printing device, more particularly to an electrostatic printer, laser printer, or other multicolor printing device in electrophotography.
As is well known, almost all colors and shades can be realized by combining the three primary colors i.e., red, green and blue or the complementary colors thereof, i.e., cyan, magenta, and yellow. Even the electrophotography multicolor printing process, comprising the steps of charging, latent image formation, development, transferring and cleaning, employs developers using the above-mentioned three primary colors or complementary colors.
One well-known conventional electrophotography multicolor printing process comprises repeated steps of latent image formation and development. Another comprises changing the latent image electric potential and development by changing the colors in accordance with the electric potential.
FIG. 1 is a schematic view of a conventional multicolor printing device employing the principle of repeated latent image formation and development. In FIG. 1, a drum 1 is formed by a conductive supporting body 1a and a photoconductive film 1b. The surface of the drum 1 is uniformly charged by a corona charger 2. A latent image with a developing color corresponding to cyan is formed on the photoconductive film 1b by a laser light source 3. The latent image formation portion is then developed by a cyan developer 4a, i.e., a cyan developing toner, by means of a developing machine 4. Next, a latent image with a developing color corresponding to yellow is formed on the photoconductive film 1b by a laser light source 5, and the latent image formation portion is developed by a yellow developer 6a by means of a developing machine 6. Similarly, a latent image with a developing color corresponding to magenta is formed on the photoconductive film 1b by a laser light source 7, and the latent image formation portion is developed by a magenta developer 8a by means of a developing machine 8.
After the cyan latent image, yellow latent image, and magenta latent image are developed, toner images are formed on the photoconductive film 1b are transferred to a paper 10 using a corona discharger 9. The residual toners on the photoconductive film 1b are then removed by a fur brush 11 to clean the photoconductive film 1b. The drum 1 is then rotated and the above-mentioned processes, i.e., charging, latent image formation, development, etc. are repeated for a continuous printing process.
However, this conventional device has the problem of mixing between the colors. FIGS. 2A to 2C are schematic views explaining this phenomenon.
As shown in FIG. 2A, after charging, cyan developing toners 12a are supplied to a latent image formation portion 12 corresponding to the cyan developer. Then, as shown in FIG. 2B, yellow developing toners 13a are supplied to a latent image formation portion 13 in accordance with the predetermined electric potential. However, as shown in FIG. 2C, when the yellow developing toners 13a are supplied to the latent image formation portion 13, part of the cyan developing toners 12a supplied to the cyan latent image formation portion 12 is sometimes replaced by excessive yellow developing toners 13b, because of electrical or mechanical forces.
Consequently, proper colors are not developed in the predetermined positions. Thus, the above-mentioned problem of color mixing occurs.
FIG. 3 is a schematic view of another conventional device employing the principle of changing the latent image electric potential and development by changing the colors in accordance with the electric potential. In FIG. 3, parts corresponding to those of FIG. 1 are represented by the same reference numerals.
In FIG. 3, a surface of a drum 1, comprised of a conductive supporting body 1a and a photoconductive film 1b, is uniformly charged by a corona charger 2. Then, half of the electric potential of portions other than latent image formation is removed by laser light source 3. Latent images of another color are then exposed by another laser light source 3 to substantially reduce the above electric potential to zero. The resultant distribution of the electric potential is illustrated in FIGS. 4A-4C. The high electric potential position is the first latent image and the substantially zero voltage portion is the second latent image. After the first and second latent images are formed, red toners, for example, are adhered to the first latent image by a developing machine 4. Then black toners, for example, are adhered to the second latent image by a developing machine 6. In this way, a two-color printing process is carried out.
It is, however, difficult to apply a middle level electric potential (Vb) to form a latent image due to factors such as deterioration of the photoconductive film or due to the laser light source. This makes it difficult to realize printing of more than two colors.